Multiple Choice
Using the appropriate bond energies, calculate the enthalpy change (ΔH) for the reaction: H_2(g) + Cl_2(g) → 2HCl(g). Given the following average bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol.
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8. Thermochemistry
Thermochemical Equations
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Nitromethane (CH3NO2), sometimes used as a fuel for drag racing, burns according to the following reaction:
4 CH3NO2 (l) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (g) + 4 NO2 (g) ∆Hº = – 2441.6 kJ
How much heat is released by burning 125.0 g of nitromethane (MW:61.044 g/mol)?
A
1.250 x 103 kJ
B
-1.250 x 103 kJ
C
2.440 x 103 kJ
D
-2.440 x 103 kJ
Verified step by step guidance1
First, determine the number of moles of nitromethane (CH3NO2) in 125.0 g. Use the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \). The molar mass of nitromethane is 61.044 g/mol.
Calculate the moles of nitromethane: \( \text{moles of CH3NO2} = \frac{125.0 \text{ g}}{61.044 \text{ g/mol}} \).
Next, use the stoichiometry of the reaction to determine the heat released. The reaction shows that 4 moles of CH3NO2 release 2441.6 kJ of heat.
Set up a proportion to find the heat released by the moles of nitromethane calculated: \( \text{heat released} = \left( \frac{\text{moles of CH3NO2}}{4 \text{ moles}} \right) \times 2441.6 \text{ kJ} \).
Finally, ensure the sign of the heat released is negative, as the reaction is exothermic, meaning heat is released.
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